In B4J is "easy" since it's CSS capable... in B4A, not so...
the way I can visualise it would be with 1 or 2 canvas and 2 panels... Set the gradients for each panel, with the solid colors you want, then set a transparent to solid gradient
on the canvas and "sandwhich" them together.... by applying the panel1 background to the canvas, and then appllying the canvas to panel2... never tried it though